1. Field of the Invention
This invention relates to a trip assembly for a circuit breaker which responds to the magnetic forces generated by overcurrents. More particularly, it relates to an arrangement for the user of the circuit breaker to easily adjust the level of overcurrent at which the magnetic trip assembly responds.
2. Background Information
A common type of circuit breaker trip unit which responds with an instantaneous trip to overcurrents such as those caused by a short circuit utilizes a magnetic solenoid. The current in the protected circuit is conducted through the coil windings generating attraction forces between a stationary and a movable armature. A spring generates a bias force opposing the attraction force applied to the movable armature. When the current magnitude achieves a level at which the magnetic force exceeds the spring biasing force, the moving armature pulls into the stationary armature. This mechanical motion is used to trip the circuit breaker. Commonly, this occurs at a discreet value of current.
It is an object of the present invention to provide an arrangement for easy adjustment of the magnetic tripping current in the field.
In accordance with the invention, an adjustable magnetic trip unit for a circuit breaker comprises a trip solenoid having a coil and a plunger movable relative to the coil. A first end of the plunger is extendable from a first end of the coil to a trip position when the coil is energized by a current of at least of a selected amplitude. The second end of the plunger extends from the second end of the coil. A spring assembly includes a spring engaging the second end of the plunger to apply a bias force to the plunger setting the selected amplitude of current, and an adjustment mechanism adjusting the bias force, and therefore, the selected amplitude of the current. The spring can be a torsion spring having a first end engaging the second end of the plunger and a second end wound by the adjustment mechanism to adjust the bias force. This adjustment mechanism includes an indexer setting discrete positions to which the second end of the torsion spring is wound to provide a plurality of discrete values of the selected amplitude of current. The adjustment mechanism also includes an adjustment knob for setting the indexer to the plurality of discrete positions and a coupler coupling the adjustment knob to the second end of the torsion spring. The coupler comprises a first bevel gear engaging the second end of the torsion spring, a second bevel gear engaging the first bevel gear, and a shaft connecting the second bevel gear to the adjustment knob for rotation by the adjustment knob. The torsion spring has an axis coincident with the pivot axis of the first bevel gear which is substantially perpendicular to the plunger. The shaft has an axis which is orthogonal to both the plunger and the axis of the torsion spring.
The adjustable magnetic trip unit is adapted to be mounted in a circuit breaker housing having a seat. The indexer is mounted on the shaft and has a peripheral cam surface engagable with the seat in the plurality of discrete rotational positions. The indexer is axially displaceable between a locked position engaging the seat and an unlocked position free of the seat in which the indexer, and therefore the shaft, can be rotated between the plurality of discrete positions. A second spring biases the indexer to the locked position. The indexer is movable against the biasing provided by the second spring by depression of the adjustment knob. The adjustment mechanism can include a gap adjuster engaging the second end of the plunger and driven by the first bevel gear to adjust the position to which the plunger is biased by the torsion spring.
In addition, the invention is directed to a circuit breaker adjustable magnetic trip assembly which includes a housing, a trip solenoid having a coil energized by load current and a plunger movable to a trip position in response to load current above a selected amplitude, a torsion spring having a first end engaging the plunger to apply a bias force to the plunger setting the selected amplitude of the load current at which the plunger moves to the trip position and an adjustment mechanism. The adjustment mechanism comprises a first bevel gear engaging a second end of the torsion spring, a second bevel gear engaging the first bevel gear, a shaft engaging the second bevel gear, an adjustment knob mounted on the shaft, and an indexer setting a plurality of discrete rotational positions of the adjustment knob. The housing has a seat and the indexer selectively engages the seat at the plurality of discrete rotational positions. The indexer has a peripheral cam surface with a plurality of flats and a seat in the housing has complimentary flat surfaces against which the indexer seats at the plurality of discrete rotational positions. The indexer is mounted for axial movement between a locked position in which it engages the seat and an unlocked position in which it and the adjustment knob are free to rotate. The adjustment mechanism further includes a bias spring biasing the indexer axially to the locked position. The housing can be a molded housing having a first slot forming the seat and an axially adjacent second slot with which the indexer is aligned in the unlocked position. The second slot is sized to allow rotation of the indexer. The housing has a third slot adjacent the second slot in which the second bevel gear is mounted for rotation. The adjustment knob and the indexer are axially movable relative to the second bevel gear which is restrained from axial movement by the third slot.
The adjustment mechanism further includes a gap adjuster adjusting an unactuated position to which the plunger is biased by the torsion spring. This gap adjuster can comprise an adjustment member coupled to and moved by the first bevel gear. The plunger can have a first inclined surface and the adjustment member can be a slide having gear teeth engaging the first bevel gear and a complimentary second inclined surface engaging the first inclined surface. The plunger can have a first end extendable from the first end of the coil to a trip position and a second end extending from a second end of the coil and having a flange with a first axially facing surface engaged by the first end of the torsion spring and a second axially facing surface bearing against the first inclined surface.